Geologic History of Johnson County, Iowa: An Emphasis on the Devonian Fossil Gorge By Lindsey Downes and Shannon Wilson Table of contents Abstract Page 2 Introduction/Statement of Purpose Page 2 Geography Pages 2­7 Economic Resource of Johnson County Page 8 Geology and Theme Page 9 References Pages 10­11 1 Abstract This following paper summarizes the geological history of Iowa and in particular Johnson County. Details can be found on the types of bedrock found in Iowa and common fossils found in Johnson County. Specific attention is given to the Coralville Lake area in Coralville, Iowa and the Devonian Fossil Gorge. The Devonian Fossil Gorge is an impressive formation displaying several fossils preserved in Devonian rock and gives an unique insight into what the ecosystem of that time period was like. The fossil gorge is later included as part of a middle school science lesson on geology, classification and ecology. Introduction ­ Statement of purpose ​ ​ The central focus of this paper is on the Devonian Fossil Gorge in Coralville, Iowa. The main focus of the student project will be identification of fossils found in the Devonian Fossil Gorge and making ecological connections between the organisms and their environment. Students will need to understand the geologic history of the Johnson County, historical flooding events that have helped to uncover the Devonian Fossil Gorge, and the organisms that existed during the Devonian Time Period. Geography Johnson County is located in Southeast Iowa (see Figure 1) and was settled in 1836. According to the United Census Bureau, the most recent estimated population of Johnson County, Iowa is 144,251 people (U.S. Census, 2015) and the population density is 213.1 people per square mile (U.S. Census, 2010). The prominent watersheds in Johnson County are The Lower Cedar Watershed, the Middle Iowa Watershed and the Lower Iowa Watershed. The Lower Cedar Watershed contains the cities of Marion, Cedar Rapids and Tipton. The Middle Iowa Watershed contains the cities of Tama, Marshalltown and Belle Plaine. The Lower Iowa Watershed contains the cities of Iowa City and Coralville and is the watershed that contains the Devonian Fossil Gorge (United States Environmental Protection Agency, 2016). Major streams and rivers of Johnson County include The Iowa River, Turkey Creek, Rapid Creek, Clear Creek, Ralston Creek, and Old Mans Creek. The source of water flowing into the Coralville Lake (which contains the Devonian Fossil Gorge) is the Iowa River. Figure 1: Map displaying the counties of Iowa. Johnson County is marked in yellow and circled. (Iowa Department of ​ Natural Resources, 1999). 2 Johnson County is characterized by several soil types (see Figure 2 below), some of the most prevalent being Colo­Ely complex (6.5% of Johnson County), eroded Ladoga silt loam (6.6%), Clinton silt loam (3.1%), Muscatine silt loam (2.0%), Tama silt loam (7.2%), Fayette silt loam (10.9%) (United States Department of Agriculture, 2015). Soil Types in Johnson County Figure 2: Map displaying various soil types throughout Johnson County. (United States Department of Agriculture, ​ 2015). Johnson County Iowa was settled in 1836 and named after Colonel Richard Mentor Johnson (Johnson County Iowa Website). It was initially settled as farmland when the country was expanding westward. Until recently much of the county was dedicated to small family farms which have converted to large­scale operations run by corporations. The development of larger cities within the county also created an urban sprawl effect which limited the available amount of farmland left. Iowa City, the largest town in Johnson County, served as Iowa’s capital until 1840­1857 (The Goldfinch, 1984) and is currently the home of the University of Iowa. The University largely contributes to the growing population of Johnson County. Because of the growing population and the fact that the 20.4% of the population is represented by those under 18 years of age (United States Census Bureau, 2014), the school districts are growing. The largest school district in Johnson County (Iowa City Community School District) is responsible 13,000 students and encompasses roughly 133 square miles (Iowa City Community School District Website, 2014). Such a large population requires city upkeep including maintaining roadways. Johnson County is responsible for over 900 miles of paved, gravel and dirt roads (Johnson County Iowa Website). There are a wide variety of businesses that are economically valuable to Johnson County, not limited to but including farming, retail, manufacturing and academics (Johnson County Iowa Website). Johnson County is home to several natural resources, the most economically valuable for the state being limestone, sand and gravel. The quarries in Johnson County, Iowa outsource their products for use in construction and road fill. As with all forms of strip mining, quarrying for 3 limestone comes with some environmental concerns. Since limestone is a high porosity rock, there is a higher potential for water carrying contaminants from the quarrying process to travel through the rock and into water sources (Bliss et. al., 2008). Figure 4: State of Iowa Bedrock Map (Iowa Geological Survey, 1969). ​ According to the bedrock map of Iowa (see figure 4), in the Northwest quarter of the state, there is a large bedrock formation that is undifferentiated. This was developed during the cretaceous period. The Cretaceous period was the longest part of the Mesozoic Era. There was an extinction after the Jurassic period. The Southwest corner of the state has Virgil and Missouri bedrock. These were both created during the Pennsylvanian period. The Pennsylvanian period was in the Paleozoic Era. It is sometime referred to as two different sub periods. In the middle of the state there are several different types of bedrock. These types of bedrock are called the undifferentiated, middle, upper and Des Moines bedrock. These different types of bedrock were created over several different time periods and include the Ordovician, Devonian, Mississippian and then the Pennsylvanian again. The Ordovician period takes place within the Paleozoic Era. This time period is characterized by significant seafloor spreading which in turn caused the temperatures of the seas and oceans to increase. During this time, North America was almost completely underwater and the sediment deposited trapped the dead sea life to form fossils. There was also a huge boom in the diversity of organisms during the Ordovician time period. Some of the species of this time period include brachiopods, trilobites, and graptolites. These organisms and several others can be found in the Devonian Fossil Gorge. The Silurian, Cambrian and Precambrian areas of bedrock can also be found in Iowa. These can all be found in the Northeast part of Iowa (Holland, 2016) . The Silurian time periods is characterized by ​ ​ rising seas levels due to glacier melt. This rise in sea level contributed to a more stable climate on Earth. This stability allowed coral reefs to form in the seas and is responsible for the large 4 diversity of ocean species of this time period. A notable development during this time period was the first vascular plant. This had huge implications for life on land (University of California Museum of Paleontology, 2009). The Cambrian Era is known for fastest growth in the number ​ of major new animal groups which happened over the course of 40 million years (University of ​ California Museum of Paleontology, 2009). The rocks near Dubuque were formed in the ​ Precambrian Era. The Precambrian Era covers 90% of the history of the Earth which is everything before 600 million years ago. There isn’t a record of life form, but even if there was life form during the Precambrian time, it probably would not have left trace fossils. It is typically divided into three different eras, the Hadean Era (which is the first one), the Archeon Era, and the Proterozoic Era.(Schaetzel) The Stratigraphic Column of Iowa is another way to look at the bedrock of Iowa. It not only gives the different time periods in which the rock was created, but it also gives an estimate as to how deep that section of rock might be. (See figure 5). Figure 5: Stratigraphic Column of Iowa (Iowa Department of Natural Resources, 2004) ​ Today, Iowa is a prairie state having gradual relief and gentle slopes. In some places the floodplain is easily seen. The Mississippi and the Missouri River’s help the state to be well drained. There are some parts of the state that show extensive erosion and other parts that have little erosion (Physiography of Iowa). Surficial formations are present in Allamakee County due to this area remaining untouched when the last glacier passed over Iowa creating glacial till and flatten the rest of Iowa (Northeast Iowa Resource Conservation and Development). 5 The primary bedrock of Johnson County dates back to the Mississippian and Devonian Time Period during the Paleozoic Era, a time period that lasted nearly 40 million years. The rock type of this period is mainly limestones, but there is also some shale found in the region. The limestone is thought to have formed while Iowa was covered by warm, shallow seas. This time period is characterized by both vertebrate and invertebrate life forms ( Rose, 1967). Several Mississippian rock formations can be seen at the Coralville Reservoir in Coralville, IA. Limestone outcrops line the shores of the man­made lake. While the primary bedrock in Johnson County is Mississippian in nature, the boundary between Devonian and Mississippian bedrock is sometimes difficult to determine. The Coralville Lake area exhibits exposed rocks and sediments from three different geologic systems: Devonian Limestone (370­380 million years old), Pennsylvanian deposits (310­320 million years old) and more recent Quaternary aged glacial and wind­blown deposits (1.5 million years old).